A beam of radiation can be separated into two parts which follow different paths and are then brought back together to form a single beam. If the two paths are not of identical optical length, the two beams may not be in phase, and can destructively interfere at some points (resulting in areas of relative darkness), and constructively interfere at other points (resulting in areas of relative brightness). A beam can be split in two wherever it strikes a stack of transparent film. While some of the beam is reflected at the upper surface of the stack, the rest of the beam enters the stack and continues through it until it is reflected at some lower film surface at which time it continues back through the stack and exits the same side it entered. The difference in path lengths for the two rays is a function of the thickness of the additional film or films that the second ray passes through. The result is a fringe pattern of areas of relative darkness and lightness which gives a contour map of the thickness of the film(s) with each contour line representing a difference in thickness of .lambda./2, where .lambda. is the wavelength of the radiation used. This contour map-like pattern is commonly known as "Newton's Rings".
These interference patterns are especially troublesome when they occur in imaging systems, such as photographic equipment and digital radiation imaging systems. Digital radiation imaging systems employ photoconductive materials to absorb incident radiation representative of an image of an object. Suitable photoconductive materials will absorb the radiation and produce electron-hole pairs (charge carriers) which may be separated from each other by an electric field applied across the photoconductor, creating a latent image at the surface of the photoconductor (which is typically a thin planar layer). A narrow beam of scanning radiation substantially completes discharge of the photoconductor by creating the motion of a second set of charge carriers. The distribution of these second charge carriers in the plane of the photoconductor is affected by the distribution of the first charge carriers, i.e., by the latent image. The motion of the second charge carriers is detected and digitized in an appropriate circuit, thereby capturing the latent image in digital form.
Various methods for minimizing the effects of interference fringes have been patented. Several known methods involve the addition of particles dispersed in or on the top surface of a layered imaging stack to diffuse light rays. Other known methods include roughening or dimpling the top surface to induce light scattering.
The above methods reduce interference fringes to varying degrees. However, such reduction is commonly accompanied by a reduction in image resolution. It would be desirable to minimize the effects of interference fringes while at the same time maintaining good image resolution.